Tidal marshes maintain relative elevation (relative to mean sea level) by trapping sediment that is suspended in flood water. Tidal marshes flood daily at high tide, and the amount of sediment that is retained by the marsh is proportional to the concentration of sediment suspended in the water, the biomass of the vegetation on the marsh surface, and the duration of the flood. In many estuaries marshes are disappearing because sea level is rising faster than the marsh can trap sediment, i.e. there is a sediment deficit. Even in the absence of sea-level rise, a sediment deficit can develop when sediment discharge from the watershed decreases or when levees are constructed to prevent flooding or to serve navigation interests. Sediment discharge from the watershed decreases when darns are constructed on the tributaries. Abandonment of agriculture and reforestation, adoption of water quality practices that reduce turbidity or even repopulation of beaver also reduce sediment discharge.
The State of Louisiana's Coastal Protection and Restoration Authority (CPRA) 2012 Coastal Master Plan is based on a two year analysis involving some of the state's best scientists as well as national and international specialists. “The state used this analysis to select 109 high performing projects that could deliver measurable benefits to our communities and coastal ecosystem over the coming decades. The plan shows that if these projects were fully funded, at a price tag of $50 billion, flood protection for communities could be substantially increased and a sustainable coast could be created. Louisiana is in the midst of a land loss crisis that has claimed 1,880 square miles of land since the 1930s. Given the importance of so many of south Louisiana's natural assets—its waterways, natural resources, unique culture, and wetlands—this land loss crisis is nothing short of a national emergency, one that takes a daily toll on the lives of coastal residents. To address this crisis the Louisiana Legislature passed Act 8 in 2006, which created the CPRA and required it to develop a coastal master plan every five years. The first master plan was approved by the legislature in 2007. The 2012 master plan was submitted to the legislature for approval on March 26th and was unanimously approved by the legislature on May 22, 2012”. The CPRA master plan proposes that marsh restoration can be done by constructing a series of diversions along the Mississippi River that essentially siphon muddy water into the bayous behind the levees or conveying sediment to restoration sites by pipelines,
However, these are highly controversial proposals because 1) each diversion is essentially a point source that supplies sediment only locally, 2) the diversions are expensive engineering works and are fixed in place, 3) diversions can alter the salinity of the estuary and disrupt traditional fisheries (e.g. oysters), and 3) the high nutrient concentration of the Mississippi River is thought by some to be harmful to the marshes.
As such, a need exists for an improved sediment management plan for the restoration and/or maintenance of tidal marsh areas.